The Martian moons Phobos and Deimos may have been created in the aftermath of a cataclysmic collision between early Mars and an object with a mass somewhere between that of the dwarf planet Ceres, and the asteroid Vesta. The theory will hopefully be put to the test in the not-too-distant future by an ambitious Japanese mission, that will seek to visit both of the small moons with the aim of retrieving a surface sample for return to Earth.

The formation of our solar system is a tale of incredible violence. This brutal past could well be reflected in the formation of Earth's moon, and in that of the legion of natural satellites known to accompany the eclectic family of planets orbiting our Sun.

For example, it is possible that some 4.5 billion years in the past, the fledgling Earth was struck by a planetary body roughly the size of present day Mars. The resultant disk of debris could then have clumped together, slowly coalescing to form the satellite we know and love today.

A new paper authored by scientists from the Southwest Research Institute asserts that the Martian moons Phobos and Deimos were created in much the same way, albeit with the involvement of a much smaller aggressor.

It had previously been proposed that the two small moons were either wandering asteroids that had been captured by Mars' gravitational influence, or satellites that had coalesced in the wake of a violent collision between the Red Planet and a large impactor.

Both of these arguments have some pretty serious flaws. For example, a large impact would create an enormous disk of debris, and while this would feed the creation of more massive moons, smaller bodies would likely be unable to coalesce. What's more, the close, roughly circular orbits of Phobos and Deimos are inconsistent with the idea that they were once asteroids that were strong-armed into joining the Martian family by the Red Planet's gravitational clout.

The scientists behind the new research turned to advanced hydrodynamical computer modelling in an attempt to identify a scenario in which the small moons could have formed in the orbit of the Red Planet. First, the team ran detailed simulations designed to reveal the nature of a circum-Martian disk of debris from which Phobos and Deimos could have coalesced.

Once this was achieved, the researchers went on to simulate collision scenarios between early Mars and a series of impactors, with variables including mass, and angle of impact, to discover the kind of a collision needed to form the debris disk identified in the first phase.

"We used state-of-the-art models to show that a Vesta-to-Ceres-sized impactor can produce a disk consistent with the formation of Mars' small moons," said Dr. Julien Salmon, an SwRI research scientist and second author for the newly published paper. "The outer portions of the disk accumulate into Phobos and Deimos, while the inner portions of the disk accumulate into larger moons that eventually spiral inward and are assimilated into Mars.

The size of the impactor proposed by the scientists is significantly smaller than had previously been considered. Vesta has a diameter of only 326 miles (525 km), while the larger Ceres is 587 miles (945 km) wide. Relative to Mars, which boasts a diameter of 4,220 miles (6,792 km), the potential impactor could be considered pretty scrawny.

According to the simulations, if the moons had formed out of a debris disk thrown out in the aftermath of such a collision, they would be predominantly composed of material that had originated on Mars. Furthermore, the moons would be extremely dry, even in the context of the Red Planet, as any water vapor contained in the ejected materials from which the moons could have formed would have been lost to space.

The theory could be put to the test by the Japan Aerospace Exploration Agency's (JAXA's) Mars Moons eXploration (MMX) mission, which is expected to launch in the early 2020s. Upon arrival at the Martian system, NNX will visit both Phobos and Deimos, and take a sample of one of the moons for return to Earth.

Should that sample be very dry, and largely similar in composition to Mars, it would strongly support the findings of the Southwest Research Institute team. Understanding how the moons of our solar system came to form significantly improves scientists' understanding of the formation processes that sculpted the planets that they now orbit. Furthermore, these insights can be taken and used to better understand the nature of exoplanets discovered beyond our Solar System.

A paper detailing the findings has been published in the journal Science Advances.

Source: Southwest Research Institute

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